the establishment of the blosk to polyspermy during fertilization is a critical event in the process of reproduction. Penetration of the egg by more than one sperm results in abnormal development and death of the embryo. In humans polyspermic conceptions account for a significant proportion of spontaneous aboutions during the firs trimester of pregnancy. Using sea urchin gametes as a model system, we have discovered a novel perozide mediated blodk to polyspermy. Fertilized sea urchin eggs released H2O2 during the cortical reaction. the Peroxide reacts with a putative sperm peroxidase to inactivate other sperm near the egg. Sperm also contain a catalase that may protect them from egg derived H202. We now propose to elucidate this process in sea urchins by determining; (1) how the fertilized egg makes peroxide(s); (2) the subcellular localization of peroxide(s) production istes by cell fractionation and electron cytochemistry; (3) whether the egg also uses other "active oxygen species" to prevent polyspermy; and (4) how the egg protects itself from the peroxide(s) it producxes. We also plan to study the effects of peroxide(s) on sperm by; (1) biochemically characterizing the sperm peroxidase and catalace and establishing their sensitivity to specific inhibitors; (2) localizing these enzymes in sperm by cell fractionationand electron cytochemcial techniques; (3) isolating the sperm catalase and peroxidase and then using fertility; and (4) determining how the peroxide-peroxidase reaction reduces sperm fertility. Our long term objective is to extend these studies to mammalian and ultimately human gametes. this could lead to significant advances in the control of human fertility.